This invention relates to the aperture structure of a zooming lens.
A zooming lens whose full-aperture diameter is varied with zooming is known in the art. A zooming lens of this type needs a mechanism for adjusting the full-aperture diameter during zooming and for also adjusting aperture diameters having JIS series F values other than the full-aperture value with zooming. Typical examples of such a mechanism have been disclosed in the specifications of Japanese Patent Application Publication No. 20911/1965, Japanese Patent Application Laid-Open No. 24134/1974 and Japanese Utility Model Application Laid-Open No. 123933/1978.
In such a conventional zooming lens, as shown in FIG. 1, an aperture blade 4 is rotated by an opening and closing ring 2, in order to obtain aperture diameters corresponding to JIS series F values other than the full-aperture value. When the aperture blade 4 returns toward the position of the full-aperture diameter after a photographing operation has been completed, the rotation of the opening and closing ring 2 adapted to operate the aperture blade 4 must be stopped at a predetermined position so that the aperture blade may not be opened more than the full-aperture diameter corresponding to the present zooming position of the lens.
However, a release plate 1 on the camera body side carries out an automatic lens stopping operation by moving in the direction of arrow A to transmit the lens stop down operation from the camera body to the ring 2 through a drive lever 5. This plate 1 is so designed that, after the photographing operation has been accomplished, the release plate 1 is moved in the direction of arrow B to an idle position which is independent of the zooming position of the lens and is beyond the predetermined position at which the aperture blade 4 is opened to its predetermined full-aperture diameter corresponding to the zooming position of the lens. This is referred to as an idle operation which is effected to positively set the aperture blade for a desired full-aperture diameter according to the relationship between the camera body and the lens. In an EE control system, the amount of movement of the release plate past its position corresponding to the full aperture to its position corresponding to the stop of the drive lever 5 is the amount of "pre-run" for adjusting exposure levels of interchangeable lenses which are different according to the types of interchangeable lenses. This operation is described in the specification of Japanese Patent Application No. 19966/1980 filed by the present applicant.
With the aperture blade set for a predetermined full-aperture diameter corresponding to the zooming position of the lens, the release plate 1 must be moved further, and therefore in the conventional zooming lens a protective spring 6 is interposed between the opening and closing ring 2 and the release plate 1 as shown in FIG. 1 so that, as the lever 5 continues to pivot clockwise after the aperture blade 4 has stopped at its maximum aperture position, the expansion of the protective spring 6 absorbs the excess stroke until the plate 1 stops at the predetermined idle position to which it continues to move.
However, since the protective spring 6 is interposed between the ring 2 and the release plate 1 as described above, the effective operating force of an aperture spring 7 is reduced by as much as the difference between the protective spring load and the aperture spring load, or by as much as the protective spring load, depending upon the position of the protective spring provided.
In a zooming lens in which the automatic lens stopping operation is controlled by allowing the plate 1 to move in the direction of arrow A and then stopping the release plate 1 after it has been moved as much as a stroke which is defined by EE control, programmed EE control or body preset control, the presence of the protective spring 6 makes the amount of rotation of ring 2 inaccurate, as a result of which the aperture operation becomes irregular and the response of the ring 2 to the release plate 1 is degraded in accuracy, all of which increases the lens stopping error. Furthermore, the inertial mass of the drive system required to drive the aperture blade is increased.
Thus, the conventional variable aperture diameter zooming lens suffers from at least the following problems:
(1) Since the aperture blade driving spring force cannot be sufficiently increased, it is difficult in the case of a lens having a large aperture diameter according to the above-described system to achieve a sufficiently fast aperture blade operating time (which is the sum of the time required for stopping the lens to a predetermined aperture value and the time during which the bouncing occurs while the lens stopping operation is damped until the aperture diameter become sufficiently stable in a certain range). Thus, the lens aperture may not stabilize before the shutter operation is effected. Thus, it is difficult to satisfactorily manufacture such a lens.
(2) Since the inertial mass of the aperture blade driving system is large, it is also difficult to manufacture a lens having a large aperture diameter variation.
(3) The aperture blade operation is liable to become irregular in the case of EE control, and therefore the aperture control error is increased.